Natural mica is notable for its unusual electrical insulating characteristics. A desire to assure a constant supply of mica material has spawned extensive research for synthetic micas.
U.S. Pat. No. 4,239,519 (Beall et al.) describes in detail several families of synthetic lithium and/or sodium mica glass-ceramics. The patent discloses that each of these families characteristically undergoes spontaneous swelling upon exposure to a polar liquid such as water. The expanded mica is easily delaminated to form a gel with the liquid.
The gel, in turn, may undergo ion exchange when exposed to certain cations larger than lithium or sodium, for example potassium ions in a salt solution. The ion-exchanged, lithium and/or sodium mica that separates is commonly referred to as floc. In accordance with the patent teaching, the latter may then be processed into paper or other desired form.
The specific teachings in the Beall et al patent are concerned primarily with lithia-containing compositions. However, the possibility of a glass-ceramic family having a sodium fluoromontmorillonoid crystal phase when crystallized is recognized.
Potassium and larger alkali metals prevent swelling of the glass-ceramics in which they occur. The lithia and lithia-soda compositions disclosed in the Beall et al. patent are technically successful, but present economic problems. In general, lithium raw materials are much more expensive than their sodium counterparts. Obviously, a composition in which the alkali metal is all sodium would be desirable from an economic point of view.
Accordingly, attention was directed toward all-soda compositions centered around the stoichiometry of sodium fluoromontmorillonoid. Spontaneous glass-ceramics resulted, but were not prone to swell in water. Hence, they did not readily undergo delamination to form a gel. It was also observed that a fluoramphibole or other type crystal phase tended to develop.